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United States Patent |
6,168,878
|
Fauteux
,   et al.
|
January 2, 2001
|
Electrochemical cell having a controlled electrode surface and associated
fabrication and chemical process
Abstract
An electrochemical cell, and an associated process, wherein the cell
includes a controlled electrode surface comprising an electrode with a
metallic current collector having a surface, an electrolyte and a reduced
additive. The invention further includes a passivating layer at the
interface between the metallic current collector and the electrolyte. The
passivating layer includes the reduced additive. This passivating layer
substantially precludes contact between electrolyte solvent and surface of
the metallic current collector to, in turn, substantially prevent gas
formation within the cell, which would otherwise result from decomposition
of the solvent upon contact with the surface of the metallic current
collector. Also, the reduced additive will likewise be substantially
precluded from generating a gas upon its decomposition.
Inventors:
|
Fauteux; Denis G. (Acton, MA);
Van Buren; Martin (Chelmsford, MA);
Kolb; Eric S. (Acton, MA)
|
Assignee:
|
Mitsubishi Chemical Corporation (Tokyo, JP)
|
Appl. No.:
|
208895 |
Filed:
|
December 10, 1998 |
Current U.S. Class: |
429/59; 429/57; 429/105 |
Intern'l Class: |
H01M 010/34 |
Field of Search: |
429/57,59,216,331,105,248
|
References Cited
U.S. Patent Documents
5529859 | Jun., 1996 | Shu et al.
| |
5626981 | May., 1997 | Simon et al.
| |
Foreign Patent Documents |
7-220756 | Aug., 1995 | JP.
| |
8-273700 | Oct., 1996 | JP.
| |
Primary Examiner: Nuzzolillo; Maria
Assistant Examiner: Dove; Tracy
Attorney, Agent or Firm: Factor & Partners
Parent Case Text
This is a Continuation-In-Part of U.S. application Ser. No. 09/178,846,
filed Oct. 26, 1998, now U.S. Pat. No. 6,045,937.
Claims
What is claimed is:
1. An electrochemical cell having a controlled electrode surface
comprising:
a first electrode and a second electrode wherein at least one of the first
and second electrodes includes a metallic current collector having a
surface;
an electrolyte including at least one solvent;
a reduced additive associated with the surface of the at least one metallic
current collector;
wherein the reduced additive precludes contact between the at least one
solvent of the electrolyte and the surface of the at least one metallic
current collector to, in turn, prevent gas formation within the cell,
which would otherwise result from decomposition of the solvent upon
contact with the surface of the at least one metallic current collector;
and
wherein the reduced additive precludes gas formation within the
electrochemical cell as a result of decomposition of the reduced additive
at the surface of the at least one metallic current collector during cell
cycling and storage.
2. The electrochemical cell according to claim 1, wherein the additive
increases the coulombic efficiency of the electrochemical cell relative to
an electrochemical cell without the reduced additive.
3. The electrochemical cell according to claim 2, wherein the first through
tenth cycle coulombic efficiency is greater than 85%.
4. The electrochemical cell according to claim 2, wherein the first cycle
coulombic efficiency is greater than 91%.
5. The electrochemical cell according to claim 1, wherein the additive
further precludes dendrite formation near the surface of the at least one
metallic current collector.
6. The electrochemical cell according to claim 1, wherein the reduced
additive is either soluble or insoluble in the electrolyte prior to
reduction.
7. The electrochemical cell according to claim 1, wherein the metallic
current collector comprises a thin metal foil of less than 25 um.
8. A process for manufacturing an electrochemical cell comprising steps of:
fabricating a first and a second electrode wherein at least one of the
electrodes includes a metallic current collector having a surface;
associating at least one electrolyte, having at least one solvent, with the
first and second electrodes; and
associating a reduced additive with at least one of the electrolyte or the
at least one electrode with metallic current collector having a surface.
9. The process according to claim 8, wherein reduced additive is insoluble
in the electrolyte.
10. The process according to claim 9, wherein the reduced additive is
applied directly onto the surface of the at least one metallic current
collector.
11. A chemical process for an electrochemical cell comprising the steps of:
fabricating a first and a second electrode wherein at least one of the
electrodes include a metallic current collector having a surface;
associating at least one electrolyte, having at least one solvent, with the
first and second electrodes;
associating an additive with the electrolyte or the surface of the at least
one metallic current collector; and
forming a passivating layer between the surface of the at least one
metallic current collector and the electrolyte; wherein the step of
forming the passivating layer includes the steps of:
charging the electrochemical cell;
reducing the additive at the surface of the metallic current collector so
that the reduced additive is insoluble with the electrolyte;
blocking the at least one solvent in the electrolyte from chemical
interaction with the surface of the at least one metallic current
collector; and
precluding gas formation within the electrochemical cell as a result of
decomposition of the reduced additive during cell cycling and storage.
12. The chemical process according to claim 11, wherein the electrochemical
cell exhibits a first through tenth cycle coulombic efficiency greater
than 85%.
13. The chemical process according to claim 11, wherein the electrochemical
cell exhibits a first cycle coulombic efficiency greater than 91%.
14. The chemical process according to claim 11, wherein additive is
insoluble in the electrolyte.
15. The chemical process according to claim 14, wherein the additive is
applied directly onto the surface of the at least one metallic current
collector.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to secondary electrochemical
cells, and more particularly, to lithium electrochemical cells having
metallic current collectors and associated fabrication and chemical
processes utilizing an in situ reduced additive which substantially
precludes gas formation within the cell as a result of decomposition of
solvents used in association with the electrolyte, and, wherein the
additive itself is substantially precluded from forming a gas from its own
decomposition during cell cycling and storage.
2. Background Art
Lithium batteries have been known in the art for several years.
Furthermore, lithium batteries using liquid, gel, polymer, or plastic
electrolytes which utilize metallic current collectors are likewise well
known. While such electrolytes have been readily utilized, problems have
been identified with respect to commercial solvents, utilized within the
electrolyte, decomposing during cell cycling and storage. In particular,
without the presence of at least a conventional additive within the cell
to form a passivating layer, the solvent reacts with the electrode
interface and partially decomposes during cycling and storage. Such a
decomposition results in the formation of significant amounts of gas which
adversely affect the cell's electrochemical performance, especially
coulombic efficiency.
Although conventional additives have been used to form a passivating layer
which substantially blocks the solvent from contact with the electrode,
problems nevertheless persist. Specifically, the additive itself undergoes
decomposition during cell cycling and storage, and, such decomposition
likewise results in the generation of significant amounts of gas within
the cell.
SUMMARY OF THE INVENTION
The present invention is directed to an electrochemical cell having a
controlled electrode surface comprising a first electrode and a second
electrode wherein at least one of the first and second electrodes includes
a metallic current collector having a surface, an electrolyte including at
least one solvent, and an additive or a reduced additive associated with
the surface of the metallic current collector. The reduced additive is
substantially insoluble in the electrolyte and is preferably reduced in
situ.
The invention also includes passivating means including the additive or
reduced additive for substantially precluding contact between the solvent
of the electrolyte and the surface of the metallic current collector to,
in turn, substantially prevent gas formation within the cell, which would
otherwise result from decomposition of the solvent upon contact with the
surface of the metallic current collector. Means are associated with the
additive or reduced additive for substantially precluding gas formation
within the electrochemical cell as a result of decomposition of the
additive or reduced additive near the surface of the metallic current
collector during cell cycling and storage. In a preferred embodiment of
the invention, the electrochemical cell exhibits a first cycle coulombic
efficiency greater than 91% and a first through tenth cycle coulombic
efficiency greater than 85%.
In yet another preferred embodiment, the invention includes means for
substantially precluding dendrite formation near the surface of the
metallic current collector.
The present invention is also directed to a process for manufacturing an
electrochemical cell comprising the steps of: a) fabricating a first and a
second electrode wherein at least one of the electrodes includes a
metallic current collector having a surface; b) associating at least one
electrolyte, having at least one solvent, with the first and second
electrodes; and c) associating an additive with at least one of the
electrolyte or the surface of the at least one metallic current collector.
The present invention is also directed to a chemical process for an
electrochemical cell comprising the steps of: a) fabricating a first and a
second electrode wherein at least one of the electrodes includes a
metallic current collector having a surface; b) associating at least one
electrolyte, having at least one solvent, with the first and second
electrodes; c) associating an additive with at least one of the
electrolyte or the surface of the at least one metallic current collector;
and d) forming a passivating layer between the surface of the metallic
current collector and the electrolyte; wherein the step of forming the
passivating layer includes the steps of: 1) charging the electrochemical
cell; 2) substantially reducing the additive near the surface of the
metallic current collector so that the reduced additive is substantially
insoluble with the electrolyte; 3) substantially blocking the at least one
solvent in the electrolyte from chemical interaction with the surface of
the metallic current collector; and 4) substantially precluding gas
formation within the electrochemical cell as a result of decomposition of
the additive or reduced additive during cell cycling and storage.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described with reference to the drawings wherein:
FIG. 1 of the drawings is a schematic representation of a prior art
electrochemical cell prior to an initial charge;
FIG. 2 of the drawings is a schematic representation of a prior art
electrochemical cell subsequent to an initial charge;
FIG. 3 of the drawings is a schematic representation of an electrochemical
cell in accordance with the present invention prior to an initial charge;
FIG. 4 of the drawings is a schematic representation of an electrochemical
cell in accordance with the present invention subsequent to an initial
charge; and
FIG. 5 of the drawings is a flow chart of the chemical process of the
present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
While this invention is susceptible of embodiment in many different forms,
there is shown in the drawings and will herein be described in detail
several embodiments with the understanding that the present disclosure is
to be considered as an exemplification of the principles of the invention
and is not intended to limit the invention to the embodiments illustrated.
Prior art electrochemical cell 10 is shown in FIG. 1, prior to an
application of an electrical charge, as generally comprising first
electrode 12, second electrode 14, and electrolyte 16. Electrolyte 16
includes solvent 18 and conventional additive 20.
Prior art electrode 10 is shown in FIG. 2, subsequent to an initial
electrical charge as generally comprising first electrode 12, second
electrode 14, electrolyte 16 and passivating layer 20'. Passivating layer
20' is formed, in part, upon association of the conventional additive with
the electrode--prior to interaction by the solvent in the electrolyte.
Although such a passivating layer substantially blocks the solvent from
contact with the electrode, it will, unfortunately, generate significant
amounts of gas as it begins to decompose during cell cycling and storage.
Electrochemical cell 110 of the present invention is shown in FIG. 3 prior
to application of an initial electrical charge, as generally comprising
first electrode 112, second electrode 114, and electrolyte 116. Electrode
112 preferably comprises a metallic current collector, such as copper.
While copper has been disclosed as a preferred current collector, any one
of a number of metal or metallic species that can be produced in a thin
foil of less than 25 um and does not form an alloy or intercalate with
lithium are likewise contemplated for use.
Electrolyte 116 includes solvent 118 and additive 120. While additive 120
is shown, for illustrative purposes only, as initially associated with
electrolyte 116, it is also contemplated that additive 120 can be
associated with first electrode 112. Such association can be accomplished
by any number of conventional techniques including, but not limited to,
spraying, rolling, or coating the electrode with the additive. When an
additive which is substantially soluble with the electrolyte, such as
succinic anhydride, is to be utilized, then it can be mixed with the
electrolyte at almost any time, also using conventional mixing techniques.
Furthermore, while additive 120 will be exemplified in the experiments
discussed in detail below as generally comprising phosphites,
carboxylates, thiophenes, and anhydrides, it will be understood that such
disclosure to specific compounds is merely illustrative of acceptable
additives, and is by no means intended to be an exhaustive compilation of
all suitable additives. Indeed, it will be understood that acceptable
additives exhibit the following characteristics: 1) they can be either
soluble or insoluble with the associated electrolyte prior to reduction,
yet substantially insoluble with the electrolyte after reduction; 2) they
can be modified to a reduced state substantially without forming a gas; 3)
they form a passivating layer on the surface of a metallic current
collector so as to substantially block the solvent within the electrolyte
from contacting the surface of the metallic current collector, and, in
turn from decomposing and generating an uncontrolled passivation layer and
gas as would otherwise occur upon interaction between the solvent and the
electrode surface; 4) they result in a cell with an increase in first
cycle coulombic efficiency relative to a cell without such an additive;
and 5) they substantially preclude dendrite formation within the cell.
Examples of just some of such additives exhibiting the above-identified
properties include, but are in no way limited to HTP, TPP, A-4, SA, DSA,
THPA, PMD, BEC, and EDTDA, each of which have the following respective
chemical structures:
##STR1##
Also, for purposes of the present disclosure, solvent 118 will be
identified as comprising an organic carbonate solvent, such as propylene
carbonate (PC) or ethylene carbonate (EC), although other commercially
available and conventionally used solvents or electrochemical systems
(such as liquid (ethers), polymer, gel, and plastic) as would be readily
understood to those having ordinary skill in the art having the present
disclosure before them, are likewise contemplated for use.
Electrochemical cell 110 is shown in FIG. 4, subsequent to application of
an initial charge, as including passivating layer 124 on surface 126 of
metallic current collector 112. As will be explained in greater detail,
the passivating layer forms as a result of reducing the additive adjacent
the interface between electrolyte 116 and surface 126 of metallic current
collector 112. As previously explained, such a passivating layer will
substantially block solvent 118 from contacting the surface 126 and, in
turn, substantially preclude the generation of gas which would otherwise
result from decomposition of the solvent. As also explained, even though
the additive itself will eventually decompose, such decomposition will not
result in the generation of any significant, if any, gas. Accordingly, it
has been found that not only is gas generation substantially eliminated,
but, that the cells coulombic efficiency can be impressively increased as
compared to cells which were fabricated without an additive of the present
invention. Details relating to such efficiency will be discussed in
greater detail below.
The process associated with the manufacture of electrochemical cell 110
(FIGS. 3 and 4), as well as the actual chemical process which occurs
within the cell upon initial electrical charging, is identified in FIG. 5,
as including the following steps: First, the initial cell is manufactured
by fabricating first electrode 112, second electrode 114, and electrolyte
116. For purposes of the present disclosure, first electrode 112 will
comprise an anode having a metallic current collector 126, and a second
electrode 114 will comprise a cathode. Of course, in a secondary cell
configuration, the anode and cathode will become interchangeable with each
other, depending on whether the cell is charging or discharging. The
particular electrolyte, as well as the electrodes, will be fabricated
using conventional techniques. Additionally, solvent 118 and additive 120,
may initially be associated with electrolyte 116. However, as previously
explained, additive 120 may alternatively, or likewise, be associated with
one or both of electrodes 112 and 114, respectively.
After electrochemical cell 110 has been fabricated, passivating layer 124
is at least partially formed by applying an initial charge to the cell.
After the initial charge is applied, additive 120 is reduced near the
interface between surface 126 of first electrode 112 and electrolyte 116.
The term "reduced" is understood not only to be a formal reduction, but
also as any alteration from the additive's original, pre-reduced state.
Such a reduction includes any modification to the chemical structure of
the additive so that it is at least substantially insoluble within the
electrolyte 116, or, alternatively associated with the surface of the
metallic current collector.
The passivating layer substantially blocks solvent 118 in electrolyte 116
from contact with the surface of the metallic current collector.
Accordingly, such blocking substantially precludes solvent decomposition,
and, more particularly, gas formation within electrochemical cell 110.
Inasmuch as solvent decomposition, which would otherwise occur upon contact
with the electrode surface, and decomposition of a conventional additive,
results in a substantial loss of coulombic efficiency, it has been found
that the use of an additive of the present invention results in an
electrochemical cell having a first cycle, and in some cases a first
through tenth cycle, coulombic efficiency substantially greater than a
cell with out such an additive.
Indeed, in support of such an increase in electrochemical performance,
several electrochemical cells were fabricated using various additives
which were subsequently characterized. The experimental method and results
are summarized herein below.
First, several electrochemical cells were fabricated wherein, the cells
comprised a copper current collector (anode), a lithium metal cathode with
a lithium metal reference electrode, and an additive (0.5-5.0% by wt.) in
a 1M LiAsF.sub.6 --PC electrolyte solution. Once fabricated, cyclic
voltammetry was used to measure how efficiently the additive formed the
passivation layer within the electrochemical cell. The fabricated cells
were cycled from 3.0 volts down to 0.0 volts in a stepped fashion. The
voltammetry results were converted to coulombic efficiency and tabulated
for ten cycles, as shown below in Table I. A number approaching 100% is
preferred.
TABLE I
% Coulombic Efficiency
Additive C/1 C/2 C/3 C/4 C/5 C/6 C/7 C/8 C/9
C/10
None 84 57 57 55 48 45 39 30 25
21
HTP(2%) 58 88 90 90 88 83 79 77 73
65
HTP(1%) 60 88 84 75 65 62 49 40 28
18
TPP 41 59 62 40 25 12 05 05 05
05
A-4 40 62 73 80 85 80 73 70 69
65
SA 91 92 93 94 91 92 93 87 89
88
DSA 28 27 28 35 30 19 25 39 25
19
THPA 55 72 69 50 33 26 18 07 07
08
PMD 50 80 57 53 58 53 45 30 23
22
BEC 54 80 65 67 71 67 67 61 53
41
EDTDA 57 75 64 62 53 41 29 20 19
13
As can be seen, although many of the above-identified additives exhibited
favorable results compared to no additive, SA, HTP(1%), and BEC exhibited
extremely favorable values for first through tenth cycle coulombic
efficiency. In addition SA exhibited an extremely high first cycle
coulombic efficiency of 91%.
The foregoing description and drawings merely explain and illustrate the
invention and the invention is not limited thereto except insofar as the
appended claims are so limited, as those skilled in the art who have the
disclosure before them will be able to make modifications and variation
therein without departing from the scope of the invention.
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